The present invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus having a white balance correcting function of correcting the white balance of a video signal obtained from the output terminal of the image pickup means.
In a conventional image pickup apparatus designed to perform white balance correction by using signals from an image sensor, the influence of a chromatic object (to be photographed) is eliminated by a known method of performing white balance correction by using only signals from a white object or an object having a color close to white or signals having possibilities of representing such an object.
In this method, for example, only the above-mentioned signals are extracted from a Y (luminance) signal and R (red)-Y and B (blue)-Y signals as two types of color difference signals obtained from a signal processing circuit in a camera, and white balance correction is performed by setting the extracted color difference signal components to be close to 0. Such a method will be referred to as a white extraction scheme hereinafter. This scheme will be briefly described below.
FIG. 1 is a graph for explaining a signal extraction range in a white extraction scheme of this type. Referring to FIG. 1, coordinates represent vectors of video signals such as television signals.
Consider an image pickup apparatus which performs white balance correction in the range of color temperatures of 3,000.degree. K. to 10,000.degree. K. Assume that correction is performed to bring a white object photographed at a color temperature of 10,000.degree. K. into a state of a good white balance. In this case, a signal corresponding to the object corresponds to a point P0 in FIG. 1. If a white object at 3,000.degree. K. is present, a signal corresponding to the object is a point P1 in FIG. 1.
In contrast to this, assume that correction is performed to bring a white object photographed at a color temperature of 3,000.degree. K. into a state of a good white balance. In this case, a signal corresponding to the object corresponds to the point P0 in FIG. 1. If a white object at a color temperature of 10,000.degree. K. is present, a signal corresponding to the object is a point P2.
That is, color reproduction changes along a thick line A in FIG. 1 with a change in color temperature of a white object.
Considering two-dimensional coordinates represented by x [=(R-Y)-(B-Y)=R-B] and y [= (R-Y)+(B-Y)=R+B-2Y] in FIG. 1, the above-mentioned change indicates that the influence of a change in color temperature on color reproduction is small in the y direction and changes in only the e direction in accordance with a change in color temperature.
Assume that white balance correction is performed within the range of 3,000.degree. K. to 10,000.degree. K. in this manner. In this case, when only signals are extracted from the range (white extraction range) indicated by the hatching in FIG. 1, all the signals from a white object or an object having a color close to white are extracted. If white balance correction is performed by using these signals, the influence of a chromatic object can be reduced, and sufficient data for white balance control can be obtained.
FIG. 2 is a block diagram showing the arrangement of a main part of a conventional image pickup apparatus designed to perform white balance control based on such a white extraction scheme.
Referring to FIG. 2, reference numeral 1 denotes an image sensor for photoelectrically converting object light incident through a lens 21 and an iris 22; and 2, a luminance/chrominance signal generator for generating a high-frequency component (Y.sub.H) of a luminance signal, a low-frequency component (Y.sub.L) of the luminance signal, a red (R) signal, and a blue (B) signal by using a signal from the image sensor 1. The R and B signals generated by the luminance/chrominance signal generator 2 are respectively supplied to gain control circuits 3 and 4. The amplification factors of the gain control circuits 3 and 4 are respectively controlled by control signals from a tracking correction circuit 17 (to be described later). The gain control circuits 3 and 4 respectively amplify the R and B signals in accordance with the controlled amplification factors.
Assume that signals output from the gain control circuits 3 and 4 are R' and B' signals, respectively. In this case, the R' and B' signals are input to a color difference signal generator 5 together with the Y.sub.L signal. Color difference signals (R-Y) and (B-Y) are then generated by the color difference signal generator 5.
These color difference signals (R-Y) and (B-Y) are input to an encoder 6 together with the Y.sub.H signal to be converted into a standard television signal. Reference numeral 28 denotes an output terminal for outputting this standard television signal.
Meanwhile, the color difference signals (R-Y) and (B-Y) are also input to an automatic white balance correction unit 20 and are respectively supplied to clamping circuits 7 and 8. In the clamping circuits 7 and 8, the DC potentials of the color difference signals (R-Y) and (B-Y) are matched with each other. The outputs from the clamping circuits 7 and 8 are input to both a subtracter 9 and an adder 10.
The subtracter 9 calculates the difference between the (R-Y) signal and the (B-Y) signal to generate a signal component in the coordinate E direction in FIG. 1. The adder 10 calculates the sum of the (R-Y) signal and the (B-Y) signal to generate a signal component in the coordinate y direction in FIG. 1.
Comparators 11 and 12 compare the y signal output from the adder 10 with reference levels THa and THb corresponding to the line segments represented by y=a and y=b in FIG. 1, respectively, and output the binary comparison outputs to an OR circuit 13. The output from the OR circuit 13 becomes a binary signal which is set at low level (Lo) only when the level of the y signal falls within the range of a.gtoreq.y.gtoreq.b, and is set at high level (Hi) in other cases. The binary signal is then supplied, as a gate control signal, to a gate circuit 14.
Meanwhile, the E signal, i.e., the (R-B) signal, output from the subtracter 9 is gated by the gate circuit 14 to be supplied to a clip circuit 15 only when the output from the OR circuit 13 is set at Lo. In the clip circuit 15, signal components, of the E signal, which correspond to the portions indicated by x&gt;c and x &lt;d in FIG. 1 are clipped or rendered nonconductive. As a result, only the signal components within the white extraction range indicated by the hatching in FIG. 1 are output to a control signal generator 16.
The control signal generator 16 outputs correction signals for controlling the average values of input signal components to be equal to reference potentials Rref and Bref corresponding to the average values of signals having good white balances. The outputs from the control signal generator 16 are supplied to a tracking correction circuit 17. In the tracking correction circuit 17, the outputs are corrected to allow white balance control along the trace of a change in color temperature. The resultant white balance-corrected signals are output, as signals Rcont and Bcont, to the gain control circuits 3 and 4, respectively.
With this arrangement, the influence of a chromatic object can be reduced to a certain degree, and white balance correction which allows acquisition of sufficient control data can be performed as compared with the case wherein only a portion having the maximum luminance is extracted as a white object.
However, in the conventional image pickup apparatus described with reference to FIGS. 1 and 2, it is difficult to obtain both good tracking characteristics with respect to an illumination condition for an object and high stability with respect to a case wherein a chromatic color enters the frame.
If, for example, a control speed is set to be high with preference being given to tracking characteristics with respect to a change in illumination condition, the white balance changes too sensitively with respect to a slight movement of an object or a change therein, thus causing discomfort to the human eye. In contrast to this, if the control speed is set to be low in consideration of stability, the tracking characteristics deteriorate with respect to a large change in photographic condition, e.g., a change from an indoor condition to an outdoor condition. As a result, a frame lacking in white balance may be photographed for a long period of time.
In the conventional image pickup apparatus described with reference to FIGS. 1 and 2, since a white balance correction value is calculated from only regions considered as achromatic regions, the following erroneous operation may be performed.
Assume that the face of a human figure enters a photographic frame with a white background, as shown in FIGS. 3A1, 3B1, and 3C1. In this case, if the state shown in FIG. 3A1 is the initial state, and only a white object is present in this state, the white balance is accurately corrected, as shown in FIG. 3A2. As a result, a signal corresponding to the coordinate point indicated by the circle in FIG. 3A2 is output.
When the face of a human figure enters the frame in this state, as shown in FIG. 3B1, even a signal corresponding to the skin color of the human figure falls within the above-described white extraction range, as shown in FIG. 3B2. As a result, the control signal generator 16 outputs correction signals Rcont and Bcont in such a manner that the average values of signal components within the white extraction range, including the signal corresponding to the skin color, are set to be equal to reference potentials Rfef and Bref. With this operation, the coordinate positions indicated by the circles in FIG. 3C2 become the positions of the signals corresponding to the skin color and the white object after white balance correction. As a result, the white background is tinged with blue, and the skin color of the human figure fades.
Such a problem is not limited to a case of a white background, and a more serious problem is posed when the background has a chromatic color such as green or red. More specifically, in this case, since the white balance is corrected by using only a color signal corresponding to the color skin, the skin color fades to a greater degree. As a result, proper color reproduction cannot be performed with respect to a frame having such a special composition.
As described above, even in white balance control based on the conventional white extraction scheme, it is difficult to obtain both good tracking characteristics and high stability and completely eliminate the influences of various chromatic objects.
FIG. 4 is a block diagram showing the arrangement of a main part of another conventional image pickup apparatus designed to perform white balance control based on such a white extraction scheme.
Referring to FIG. 4, reference numeral 51 denotes an image sensor for photoelectrically converting object light incident through a lens 71 and an iris 72; and 52, a luminance/chrominance signal generator for generating a high-frequency component (Y.sub.H) of a luminance signal, a low-frequency component (Y.sub.L) of the luminance signal, a red (R) signal, and a blue (B) signal by using a signal from the image sensor 51. The R and B signals generated by the luminance/chrominance signal generator 52 are respectively supplied to gain control circuits 53 and 54. The amplification factors of the gain control circuits 53 and 54 are respectively controlled by control signals from digital-analog (D/A) converters 93 and 94 (to be described later). The gain control circuits 53 and 54 respectively amplify the R and B signals in accordance with the controlled amplification factors.
Assume that signals output from the gain control circuits 53 and 54 are R' and B' signals, respectively. In this case, the R' and B' signals are input to a color difference signal generator 55 together with the Y.sub.L signal. Color difference signals (R-Y) and (B-Y) are then generated by the color difference signal generator 55.
These color difference signals (R-Y) and (B-Y) are input to an encoder 56 together with the Y.sub.H signal to be converted into a standard television signal. Reference numeral 78 denotes an output terminal for outputting this standard television signal.
Meanwhile, the color difference signals (R-Y) and (B-Y) are respectively input to analog-digital (A/D) converters 80 and 81, and the Y.sub.H signal is input to an A/D converter 82. Outputs from these A/D converters 80, 81, and 82 are respectively input to switch circuits 83, 84, and 85. The switch circuits 83, 84, and 85 are controlled in accordance with a sync signal S1 from a sync signal generator 95 and a region designation signal S2 from a correction signal calculation section 92, and supply A/D-converted outputs corresponding to designated regions, of 64 regions into which one frame is divided, as shown in FIG. 5, to adders 86, 87, and 88, which are connected to the output terminals of the switch circuits 83, 84, and 85, respectively.
Even in a case wherein only color signals from a white object or an object having a color close to white are to be used, or in a case wherein color data formed from a color signal component from each region is to be weighted, the integral value of a color signal must be obtained for each of the 64 regions shown in FIG. 5. Therefore, the correction signal calculation section 92 must sequentially designate all the regions, ranging from a region W11 to a region W88 in FIG. 5.
Operations of the adders 86, 87, and 88 and hold circuits 89, 90, and 91 will be described next. The hold circuits 89, 90, and 91 are respectively connected to the output terminals of the adders 86, 87, and 88 so that outputs from the adders 86, 87, and 88 are temporarily held in the hold circuits 89, 90, and 91. The data held in the hold circuits 89, 90, and 91 are respectively input to the adders 86, 87, and 88 again to be added to the next A/D conversion values. That is, a digital integrator is constituted by a set of a hold circuit and an adder.
When the hold circuits 89, 90, and 91 complete integration (accumulation) of A/D conversion values corresponding to one region, the correction signal calculation section 92 loads the held data, i.e., the integral value. The hold circuits 89, 90, and 91 are reset by a reset signal S3 output from the correction signal calculation section 92 immediately after this operation.
When the integral value of the (B-Y), (R-Y), and Y.sub.H signals associated with the region W11 in FIG. 5 is to be obtained, this integrating operation is completed at time t1 in FIG. 5. That is, this integration requires a period of time almost 1/8 the vertical sync period. The correction signal calculation section 92 loads the integral values held in the hold circuits 89, 90, and 91 at time t1, and outputs the reset signal S3 to the hold circuits 89, 90, and 91.
In this case, since a short time ts is required to designate this reset operation and the next region, the region which can be designated next is the region W13 or the subsequent region. For this reason, the maximum number of regions from which integral values can be loaded by the correction signal calculation section 92 in one vertical sync period is four. That is, as shown in FIG. 6, if the integral value of the respective signals in the regions W11, W13, W15, and W17 are loaded in the first field, the integral values of the respective signals in the regions W12, W14, W16, and W18 are loaded in the second field. In the third field, the integral values of the respective signals in the regions W21, W23, W25, and W27 are loaded. In this manner, the integral values of the respective signals in the 64 regions of the entire frame are sequentially loaded.
In the image pickup apparatus in FIG. 4, therefore, a period of time corresponding to 16 fields is required to load the integral values of the respective signals in all the regions.
In the correction signal calculation section 92, after the integral values of the respective regions are loaded in this manner, these data are weighted depending on whether the data correspond to a white object, thus obtaining the average values of the data associated with the entire frame. That is, an average value Ravr of the integral values of the (R-Y) signals corresponding to the 64 regions is calculated, and an average value Bavr of the integral values of the (R-Y) signals corresponding to the 64 regions is calculated, and signals (white balance correction values) Rcont and Bcont for controlling the gain control circuits 53 and 54 are calculated such that the average values Ravr and Bavr become equal to reference values Rref and Bref for white balance correction. In this case, the reference values Rref and Bref are equivalent to the average value of the (R-Y) signals with a proper white balance and the average value of the (B-Y) signals with a proper white balance, respectively.
The white balance correction values Rcont and Bcont serve to control the gain control circuits 53 and 54 through the D/A converters 93 and 94, thus performing white balance correction.
In the above-described image pickup apparatus, white balance correction values cannot be calculated unless all the integral values of the respective signals from the plurality of regions into which the frame is divided are loaded.
The white balance correction speed, therefore, is limited by the time required to load the integral values of all the divided regions of the entire frame. For example, if the frame is divided into 64 regions, a 16-field period is required to load the integral values of the divided regions of the entire frame, as described above. When the field frequency is 1/60 seconds, a white balance correction value is updated 3.74 times at maximum within one second. As the number of divided regions of the frame is increased, the time required for this update processing is further prolonged.
For this reason, when the white balance must be caused to follow an abrupt change in color temperature or the like, since the correction speed is limited, the white balance may not be caused to follow the change in color temperature within a short period of time.